1. Field of the Invention
The present invention relates to malfunction diagnosis systems for diagnosing malfunctions in electronic control units and devices connected thereto on vehicles. More particularly, the present invention relates to a malfunction detection system for electronic control units that are connected with one another by a communications network.
2. Description of the Related Art
Various functions of vehicles are often electronically controlled. This type of vehicle employs electronic control units (ECU). ECUs are categorized as engine control ECUs, transmission control ECUs, antilock brake system (ABS) ECUs and others. Each ECU sends command signals to the corresponding device for controlling it. Inspection is conducted before shipping to judge whether the ECUs are functioning normally. Each ECU stores data regarding the device it controls. The data stored in ECUs allows the cause of vehicle malfunctions to be diagnosed.
The diagnosis of the ECUs is generally performed by an external vehicle diagnostic tester. The diagnostic tester is connected to communication ports of each ECU, through which each ECU and the diagnostic tester exchange data. In prior art systems, the transmission rate is as slow as 125 bit/sec (bps). Since the CPU incorporated in an ECU can read the data at this rate, the ECU needs no communication ICs to be incorporated therein.
However, recent data transmissions are performed at faster transmission rates. The California Air Resources Board (CARB) requires 10.4 kbps for diagnosis communications between an external vehicle diagnostic tester and an engine control ECU. At such a high transmission rate, the CPU in an engine ECU cannot read data and therefore needs to have a communications IC.
A plurality of ECUs mounted on a vehicle are often connected with one another, thereby forming a communications network. The ECUs perform mutual data transmissions using the communications network. In Japan, the transmission rate of 9.6 kbps is generally used in the network. An engine ECU that is connected to the network needs to be coordinated to the transmission rate of 9.6 kbps.
As described above, the required transmission rate for the engine ECU is different between the United States and Japan. This increases the development expenses of ECUs and complicates the product control of ECUs. A vehicle manufactured in North America usually has an engine ECU that is compliant with the local standard. When such a vehicle is imported to Japan, its engine ECU cannot be inspected by an external vehicle diagnostic tester that is compliant with the Japanese standard.
Therefore, there is a demand for an engine ECU that is adapted to both 10.4 kbps and 9.6 kbps standards. One way to meet the demand is to provide suitable hardware, that is, an ECU having two communication ICs for two different transmission rates. This increases the manufacturing cost of the ECUs. Another way is to provide an ECU with a single IC having a software program to switch data transmission rates.
An ECU that chooses a data transmission rate by a program performs data transmissions by normal I/O processing until it selects a data transmission rate. If a plurality of ECUs constitute a communications network, a diagnostic tester designates an ECU and establishes a data transmission with it.
The International Organization for Standardization (ISO) recites in ISO9141 that an initialization process for "designation of a desired ECU among others (not a transmission rate selecting method)" be performed at 5 bps, which is fairly slow.
With a transmission rate of 5 bps, it takes about two seconds to transmit one byte of data. The transmission rate is selected after an ECU among ECUs forming a communications network is designated. Therefore, under the ISO standard, selecting of the transmission rate takes a relatively long time. Designation of an ECU can be performed without the ISO standard for speeding the initialization process. However, it is likely that the ECU designating method will be required to comply with ISO standards in the future.
Therefore, there is a demand for a system that designates an ECU based on the ISO standards and selects the transmission rate for data transmission with the designated ECU. It is also desired that the period of time needed for selecting the transmission rate be shorter.